Jörg R. Hörandel Homepage Joerg R. Hoerandel
Development of finely segmented liquid ionization chambers for radiation therapy

Diploma thesis Katja Eberle
PhD thesis Thomas Berghöfer
Liquid ionization chambers are developed to be used as monitors in radiation therapy. The development focuses on two areas of application, an on-line monitor for intensity modulated radiation therapy and the measurement of the energy deposit of heavy ions in tissue. This requires a thin detector with two-dimensional read-out and an absorbing chamber with three-dimensional read-out, respectively.

The work is done in collaboration with the German Cancer Research Center (DKFZ) in Heidelberg (Prof. Dr. G. Hartmann).

Both detectors are realized using similar techniques. Ionizing radiation is measured with liquid ionization chambers, which are operated at room temperature. The liquids isooctane, isononane (TMP), and tetramethylsilane (TMS) are used in a high purity grade in order to realize high current signals. Finely segmented read-out electrodes with typically several hundred individual channels are utilized. They are read out in parallel at frequencies exceeding 10 Hz.

The figure depicts a read-out electrode with 400 channels.

Chamber with two-dimensional read-out

A chamber with two-dimensional read-out has been designed to test the liquids and to gain experience with the principal components of an envisaged monitor chamber. The dimensions and the frame have been chosen to be used gantry mounted with a miniature multileaf collimator (ModuLeaf, MRC Systems GmbH, Heidelberg, Germany) and a X-ray beam of a linear accelerator (PRIMUS, 6/15 MV, Siemens OCS, Concord, California, USA) at the DKFZ. The aperture of the collimator is 70 x 84 mm2. The entrance and exit windows are of special inert plastic material (VECTRA), which with its low mean atomic number is sufficiently transparent for X-rays and is flexible enough to allow for temperature and pressure variations.

The picture shows the prototype before being filled with ionization liquid.

The distance between the electrodes has been chosen to 5 mm in order to maximize the signal and to minimize the electronic noise. The voltage (up to 5 kV) is applied to the cathode which consists of a thin nickel mesh. The pad structure of the anode is realized as a silver palladium conductor on a ceramic substrate. The pad rows are aligned with the collimator leafs and the pad size of 3.3 x 4.0 mm2 and the spacing of 0.2 mm is adapted to the leaf width of 1.75 mm. The positions of two adjacent leafs are controlled by one row of pads. The 400 pads are surrounded by a guard electrode (3.5 mm wide rectangular frame) in order to form at the edges an adequate homogeneous electric field.

The figure illustrates an electrode configuration for a chamber with two-dimensional read-out.

Electronically the anodes are on virtual ground and the field shaping electrode is on ground potential. Each pad is connected to a circuit track on the rear side of the ceramic substrate. The photo shows the circuit track system which traces the signals to four feed-through plugs. The tracks have a width of 125 micron and approach each other 250 micron at most.
The chamber with two-dimensional read-out has been tested successfully using beams from a 60Co source and an electron linear accelerator at the DKFZ.

The current image of an asymmetrical cross formed with a multi leaf collimator is shown. The insert represents the collimator aperture: the broad arms are 7 mm wide (corresponding to 4 leafs) and the thin arms have a width of 3.5 mm.

In conclusion, it can be stated that the chamber exhibits stability of performance and shows uniformity in sensitivity and read-out.

Chamber with three-dimensional read-out

The ionization liquids used exhibit similar physical properties as human tissue. Therefore, a liquid ionization chamber can be used to measure the penetration of the radiation into the body in a phantom.

The measurement of the energy deposit as function of the depth in a medium requires a monitor with three-dimensional read-out. To test this approach a chamber with four planes of read-out electrodes has been built. Each layer contains 10 x 10 pads with the dimensions 2.5 x 2.5 mm. Two pairs of anode layers are arranged as sketched in the figure. In each layer pair the anode pads are facing each other. The high voltage is applied to two nickel grids, arranged in the center plane of each pair of anode layers. The distance between the nickel grid and the read-out layers has been chosen to be 4 mm.

The materials used and the read-out electronics are identical to the set-up of the chamber with two-dimensional read-out.

The photograph depicts the chamber with the four vacuum feed-troughs for the signals of the 400 individual channels, arranged around the beam entrance window in the center of the chamber lid. in the

The photograph shows the set-up for the first two layers of the chamber with three-dimensional read-out.

First tests of this chamber have been conducted at a 60Co-source at the DKFZ in Heidelberg. It could be demonstrated that a homogeneous read out can be achieved with this device. To verify the capability to measure the Bragg peak it is planned to test this chamber at a 12C-beam at the GSI in Darmstadt.

A liquid ionization chamber as monitor in radiation therapy
Proceedings of the 9th Conference on Astroparticle, Particle, Space Physics, Detectors and Medical Physics Applications
First tests of a liquid ionization chamber to monitor intensity modulated radiation beams
Physics in Medicine and Biology
Liquid ionization chambers for radiation therapy
Medical Imaging Conference 2003
Diploma Thesis K. Eberle (in German)
Flüssigkeitsionisationskammern in der Strahlentherapie
(Liquid ionization chambers in radiation therapy)
Nachrichten, Forschungszentrum Karlsruhe

Jörg R. Hörandel